The present invention relates to a method for performing a surface treatment upon a substrate metal which is an alloy of aluminum, and more particularly relates to a method for forming a composite layer upon the surface of a substrate metal, said layer consisting largely of aluminum alloy and ceramic particles of a carbide of a metal mixed in therewith, which the composite layer is formed by the use of a laser.
In order to improve the surface qualities of a portion of the surface of a quantity of a substrate metal material such as aluminum alloy, such as for example in order to improve the corrosion resistance, the heat resistance, or the wear resistance of the surface of a member at least largely made of such substrate aluminum alloy metal material, it is per se known to be effective to disperse ceramic particulate material into said surface portion of said substrate aluminum alloy metal material, since such ceramic particles typically have per se high strength and high hardness and are superior with regard to the qualities of heat resistance and corrosion resistance. Thereby, a surface layer is formed on the aluminum alloy substrate, said surface layer consisting largely of aluminum alloy and ceramic particles of said ceramic mixed in therewith. One way in which this surface alloying has been performed in the prior art has been to dispose a quantity of the appropriate ceramic powder material on the surface of the substrate aluminum alloy metal material, and then to fuse together said ceramic powder material and said substrate aluminum alloy metal material by heating by the use of a high energy source such as a laser, a TIG arc, or an electron beam. After the fused portion has solidified, a composite layer is thus formed, said composite layer, if the parameters of the process are appropriate and correct, having the appropriately enhanced surface properties.
However, this type of process is fraught with problems which will now be outlined, and in practice these problems have doomed all trials for thus forming a composite surface layer consisting of aluminum alloy and ceramic particles to failure.
The first reason for the above fact is thought to be that, since surface heating by a laser occurs substantially locally over a relatively short period of time, the molten metal layer solidifies without being well stirred up.
The second reason for the above is thought to be that, since such ceramic particles typically have poor wettability with regard to such molten aluminum alloy, said ceramic particles are very difficult to be well dispersed in the molten aluminum alloy.
The third reason for the above is thought to be that, since such ceramic particles typically have higher specific gravity than such molten aluminum alloy, when as described above said ceramic particles are attempted to be dispersed within such aluminum alloy, they tend to gather at the bottom portion of the molten aluminum alloy mass, due to the difference in the specific gravities.
Furthermore, as a fourth reason for problems to occur, if the ceramic particles to be dispersed in the surface layer on the aluminum alloy substrate material are particles composed of a metal oxide such as Al.sub.2 O.sub.3, SiO.sub.2, or the like, or are particles of a metal nitride such as Si.sub.3 N.sub.4, AlN, or the like, then some of said ceramic particles will decompose in the molten aluminum alloy substrate metal layer when they are heated to high temperature as by a laser. For example, ceramics composed of metal oxide tend to decompose to produce oxygen, and similarly ceramics composed of metal nitride tend to decompose to produce nitrogen, and these gases thus produced, which are evolved, tend to create blow holes or pin holes in the composite material layer finally produced. Therefore, in the case when a composite material layer is to be formed on the surface of an aluminum alloy substrate by the method described above, if the ceramic particles to be dispersed in said composite material layer are composed of an oxide of a metal or of a nitride of a metal, it is very difficult to form a composite layer in which the ceramic particles are uniformly dispersed, said composite layer having an acceptably low level of defects such as blow holes.